How about this; Put a "transaction redistributor" layer in wallets, so that users can either choose miners to send their transactions to, maybe simply based on presumed geographical node positions and hashrate, or else have a third party handle it for them. The third party is trusted to have them mined in a decentralized manner. They might certify miners, in order to verify non-collusion, but how the third party ensures decentralization is open to competition.
If the transaction doesn't verify over a certain period of time, or if users no longer trust a third party for any reason, users could always fall back to standard broadcasting of transactions. This way, users can try enforcing a decentralized mining process, even though it would mean slower confirmation times, and slightly more expensive fees.
For this to have any chance of working, miners would have to be primarily funded through transaction fees. I find that interesting because it's a separate reason why funding miners through inflation is improper, and why a robust fee market is desirable. I previously disliked funding through inflation only because a good medium of exchange should have a stable supply, for easier economic calculation, and the inflation rate isn't found through a market process.

I think this would be a difficult product to get users to adopt, because there doesn't appear to be any immediate benefit to them. It is also hard to establish trust with a third party, due to the nature of it being surprisingly difficult to prove nodes are decentralized, and therefore also difficult to ensure third parties are being effective.

Nonetheless, it is at least a method to incentivize decentralized nodes, and doesn't seem likely to make the situation worse.